If there's one thing most Republicans and Democrats can wholeheartedly agree on, it's that teenage girls shouldn't have babies. But while the former preaches abstinence and the latter comprehensive sex education and access to contraception, neither party has been able to lower the teen birthrate in the US, or figure out why it's much higher than that of other developed countries.

A new paper called "Why is the Teen Birth Rate in the United States So High and Why Does It Matter?" published in the spring issue of the Journal of Economic Perspectives by Melissa Kearney and Phillip Levine, argues that the problem is not that having babies makes teenage mothers poor; it's that poor teenagers are more likely to have babies. In their words, "being on a low economic trajectory in life leads many teenage girls to have children while they are young and unmarried and that poor outcomes seen later in life (relative to teens who do not have children) are simply the continuation of the original low economic trajectory."

It is a mistake to the leap from the observation that women who gave birth as teenagers are poor to the view that they're poor because they gave birth. Lexus owners are much richer than the average American, but that doesn't mean the average person can get ahead by buying a Lexus. Women with better economic opportunities tend to do a good job of avoiding childbirth.

Kearney and Levine came to their conclusion by studying data on miscarriages, which allowed them to analyze the reasons why one would choose to give birth, not what one's life is like after the baby is born. They found that both teenage girls who gave birth and teenage girls who miscarry don't turn out so well — and when you compare teen mothers to their own sisters who don't have babies, there's not much of a difference, either.

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That may be why, as the researchers found, few policies actually affect the teen birth rate: abortion policies don't do the trick, but neither does sex ed. What it really comes down to are state-by-state demographics — and, interestingly, low-income young women who grow up in economic inequality are even more likely to be teen moms, because they feel frustrated by the barriers preventing them from achieving the same rewards as their middle class neighbors. "Where poor people can see that hard work and 'playing by the rules' will reward them, they're pretty likely to do just that," Yglesias writes. "Where the system looks stacked against them, they're more likely to abandon mainstream norms."

The optimistic takeaway from this study is that we can all finally stop squabbling over whether abstinence or sex ed will lower the teen pregnancy rate. Yay! But pessimists — and realists — could argue that that obviously won't ever happen, because it's a lot harder — and more expensive — to think about how to improve the lives of low-income women than it is to give out condoms or pressure girls to stay virgins.